High-resistivity semiconductor radiation detectors are widely used for detecting ionizing radiation due to their ability to operate at room temperature, their small size and durability, and other features inherent in semiconductor devices. Such detectors are used in a wide variety of applications, including medical diagnostic imaging, nuclear waste monitoring, industrial process monitoring, and space astronomy. Ionizing radiation includes both particulate radiation, such as alpha or beta particles, and electromagnetic radiation such as gamma or x rays.
Conventional semiconductor radiation detectors can be configured in several types, for example, planar or monolithic. The simplest form is a planar detector 1 which includes a semiconductor crystal 3 located between cathode 2 and anode 4 electrodes, as shown in FIG. 1A. To use the detector as an imaging device, the anode electrode should be fabricated into segments, such as pixels as shown in FIG. 1b. The device in the former case is generally referred to as conventional pixilated detector. In FIG. 1b, the architecture of such conventional pixilated detectors 6 typically consists of a slab of semiconductor crystal 3 with metal covering two opposing surfaces of the slab to form two electrodes, a cathode 2 and anode pixels 7. There are various configurations to apply a field 5 across the electrodes including applying an external voltage from an external voltage source (not shown) to either the pixilated anodes 7 or the cathode 2, or both. For example, a pixilated anode 7 may be connected to external signal processing circuitry (not shown) and to ground, and the cathode 2 is connected to an external voltage source (not shown). The bias voltage across the electrodes 2, 7 results in an electric field distribution. Electron and hole “charge clouds” generated within the semiconductor crystal 3 by an ionizing radiation event A absorbed within the slab of semiconductor crystal 3 are swept toward the anode 7 and cathode 2 electrodes, respectively. These moving electron and hole clouds create charge-pulse signals in the external signal processing circuitry (not shown).
If all the electrons and holes generated by the ionizing radiation A reach their respective electrodes (i.e., the electrons reach the anodes 7 and the holes reach the cathode 2), the output charge signal will exactly equal the charge from the energy deposited within the crystal 3. Because the deposited charge is directly proportional to the energy of the ionizing radiation A, the semiconductor radiation detector 6 provides a means for measuring the energy of the ionizing radiation A. The ability to measure this energy is an important function of radiation detectors.
Conventional pixilated radiation detectors, however, suffer from a serious drawback: because of limited field confinement of known electrode structures, some of the electrons and holes near the edge are generally lost by leaking to the side surface as they sweep toward their respective electrodes. The result of this drawback is poor charge collection efficiency for the outer pixels, and increasing side surface leakage current. This effect is particularly acute in the so-called edge (outer) pixels and evident even in the first inner pixels. It is desired to have an array of anode pixels with near-identical high performance, for improved imaging accuracy.